1. Field of the Invention
The present invention relates to metal-oxide based memory devices and methods for operating such devices.
2. Description of Related Art
Some metal-oxides can be caused to change resistance between two or more suitable ranges by application of electrical pulses at levels suitable for implementation in integrated circuits. Metal-oxides have generated interest in use in resistive random access memory (RRAM) devices because of their simple structure, compatibility with standard CMOS processes, high speed, low power consumption, and potential for 3D stacking.
Tungsten oxide WOx based RRAM has been shown to exhibit good resistive switching characteristics between two or more resistance ranges. See, U.S. Pat. No. 7,800,094 entitled “Memory Devices Having an Embedded Resistance Memory with Tungsten Compound and Manufacturing Methods,” filed 12 Dec. 2007.
It is important to maintain a relatively large resistance window between the resistance states in order to reliably determine the stored data value. However, due to variations in materials, manufacturing processes, and the operating environment, the pulse characteristics (e.g. voltage amplitude, pulse width, etc.) required to change the resistance state of a metal-oxide memory cell will vary across an array.
One attempt at addressing the variation in required pulse characteristics, involves applying a single high voltage pulse capable sufficient to program each memory element to the desired resistance state. However, this results in at least some memory elements experiencing significantly higher voltages than are necessary to cause the transition to the desired resistance state. Over time, these unnecessarily high voltages can cause variations in the resulting resistance of the metal-oxide material. These variations reduce the resistance window, resulting in data reliability issues and possible failure of the device.
It is therefore desirable to provide methods for operating metal-oxide memory devices which address the endurance issues discussed above and result in improved reliability.